Does anyone care to ask an amplifier designer a technical question? My door is open.

RM

first thanks for the input on Ralphs amp, speakers likely the dead ESL-63 I mentioned in another post. I am away from them but I am assuming with ago all the panels are toast, the limited fault tree look I have done so far indicates EHT power supply issues. I am away from them so do not know on panel color, will check that when home next. Thanks all for inputs on rebuilders.

I am away from my toolbox also but I will get DMM peak voltages measured today, SPL at 1 M

fun, real data will set you free was our mantra at work for 30 years...

Hears what JA had to say in Stereophile.. The Audio Research Reference 75 measures well for a classic tube amplifier design with a single pair of output tubes for each channel and a modest degree of loop negative feedback. Its output transformers are also of good quality, the only proviso being that the amplifier should not be used with loudspeakers whose impedance drops significantly below the nominal value of the output transformer tap
Read more at https://www.stereophile.com/content/audio-research-reference-75-power-amplifier-measurements#jbpCd1H...

I lack 99% of your engineering expertise, but I don't read the measurements as being as bad as you seemingly do. It seems to me that JA has a tendency to disagree with his own reviewer's perceptions as to which output tap is best rather than criticizing the design of this amp and it's output tap measurements per se. In some ways, this entire discussion is premised upon design expectations; should the amp designer account for every possibility or to some degree, is the end-user responsible for matching the amp to a suitable loudspeaker? It seems to me that you adopt the former view, which is somewhat quizzical given your avowed distaste for dynamic coned speakers and your view that ESLs/Planars are the only form of speaker that makes musical sense. For better or worse, at Stereophile high-dollar tubed amps seem to be always matched up with various iterations of Wilsons, none of which present graceful impedance curves. I find it curious in general, Roger, that you defend measurements of tube amps as meritorious in a vacuum (pun intended). You seem to  imply that the measurements may not tell the complete story, but that just the same any mediocrity in the measurements that are utilized is sure to correlate with deficiencies in sound. In other words, I interpret your posts-not just in this thread but in others too-to be consistent with a person who adopts only the first half of the golden oldie phrase, "not all things that count can be measured and not all things that can be measured count". 
This may dismay you quite a bit from an engineering standpoint, but I take measurements of tube amps to be very analogous to measurements of DAC's; the best measuring DAC's don't often sound the best. They don't even necessarily render the so-called "musical truth" the best. Is there a known measurement of tubed gear that can predict a tube amp's ability to render the texture of a violin or the blat of a trumpet, the subtlety of David Rawling's plucking of a Martin acoustic, let alone sound stage width or depth? 
I do agree with you that for the price charged, ARC has no excuse for mounting tube sockets directly onto the PCB. But to my knowledge, that is a reliability issue and not a performance issue. I also agree with you that ARC gear is unnecessarily complex and hard to service. The same could be said for BAT and Lamm and yet they have great reputations. The sheer number of capacitors in both my ARC Ref 150SE and Ref 6 is either alarming or impressive depending upon one's point of view. Again, BAT and Lamm seem to adopt the same approach. 
Your own decision to go hybrid with your higher powered amp and to go true balanced with your higher powered amp but not your lowered power amp is a head-scratcher. If-as you state-monos are more susceptible to hum than stereo amps, why do you implement RCA-only vs. offering true balanced in the opposite direction? And while a solid state input stage may very well offer better measurements, where is the proof that it sounds better? At the end of the day, isn't it indisputable that it is cheaper to produce and less complicated? You on the one hand have little good to say about Rogue and yet when it comes to hybrid tube amps, I think of Rogue (and Musical Fidelity though I don't count their "tubes" as tubes). 

Is it harder to design a amp or preamp?

I would like to hear how each of you figured out how much power you needed to buy in watts?

Never bought by power. Always purchased by sound.
Currently have 100 and 150w SS and 40w tube

84db 1w / 1m. min 3.2Ω, max 16Ω, mean 7Ω

Loud level is 90db C weighted slow measured with REW at seating pos 13ft from LS. Don't have low level. Each selection is listened at optimum for type. Sonic wallpaper is MP3 over internet in MONO, MP3 is intolerable in stereo.

SS amps not rated @ 4Ω

Never bought by power. Always purchased by sound.

On several occasions with lackluster sound at higher SPLS, I ended up purchasing more power elsewhere and that has worked.

HI, Roger here with a question.

I would like to hear how each of you figured out how much power you needed to buy in watts?

I would appreciate the following information in your response.

Your listening level LOUD SPL (preferably measured at 1 meter from the speaker with a REAL SPL meter. Your low listening level. If you are using a cell phone app then you have confirmed it?

Your speaker sensitivity?

Listening Distance from speakers?

How many watts at your load is the amp is rated to supply?

I figured it out by listening and trial and error over time and based essentially on those parameters as a guide. However specifications alone seldom tell the whole story. The devil is always in the details much of which is never specified. Detailed measurements like those in Stereophile help a lot but I have found there may still be surprises playing real music even with very comprehensive test measurements at ones finger tips, though if done correctly, measurements certainly help with the decision making process of what to try next or not.